1. Field
This invention relates to improvements in ferromagnetic filters and, in particular, to a method for increasing the bandwidth while reducing spurious responses in such filters.
2. Prior Art
A ferromagnetic filter in its rudimentary form, consist of a coupling loop positioned close to a sphere of a ferromagnetic material located in a magnetic field. The ferromagnetic material, usually yttrium-iron-garnet (YIG), can be made to produce a principal resonance at a frequency determined by the strength of the magnetic field. The resonant frequency is given by fo=.gamma.Ho where Fo is the resonant frequency, .gamma. is the gyromagnetic ratio, 2.8 MHz/oersteds, and Ho is the applied magnetic field in gauss.
Bandwidths of such devices are usually small, typically less than 35 MHz about the center frequency. Increasing the bandwidth of these filters has been a long-standing objective. In prior art approaches to produce wider bandwidths, the coupling between the loop and the sphere has been increased by bringing the loop closer to the sphere or a strap with lower inductance has been substituted for the more commonly used single wire loop. These approaches have resulted in an increase in both crossing and tracking spurious responses. Tracking spurious are spurious responses which follow the principal resonance of the sphere as it is tuned across a frequency range. Crossing spurious are those which do not tune at the same rate as the principal resonance mode and therefore cross through the principal resonance mode as it is tuned through a frequency range.
Prior approaches designed to reduce the spurious responses have included decreasing the unloaded Q of the YIG sphere by means of roughening the surface of the sphere; however, this process increases the insertion loss at the principal resonance.
There are a number of additional problems associated with prior art YIG filters which may be explained with reference to a rudimentary YIG filter, such as that shown in FIG. 4. In this Figure, an input port 401 is connected to an output port 402 by way of a coupling loop 403, which surrounds a YIG sphere 404. Note that in all figures, dots on a line about a YIG sphere represent the initiation or termination of a coupling loop. For example, in FIG. 4B, the dots represent the initiation and termination of loop 403. In the operation of this device, a signal placed on the input port is transmitted to the output port. Signals which are at the principal resonance of the sphere are rejected and returned to the input port. In this mode of operation, the device functions as a band-stop filter.
The length of the line from the input port to the output port forms an inductance that is an integral part of the band-stop filter. This inductance limits the range over which the band-stop filter can operate because it functions as a portion of a separate low-pass filter structure. Increasing the inductance reduces the high frequency cutoff of the low-pass filter which, in turn, limits the high frequency response of the YIG filter.
One prior art approach, intended to increase the bandwidth of the YIG filter at its principal resonance, is to lower the external Q of the sphere and loop by increasing the coupling between the two. This is done by increasing the turns of the coupling loops about the sphere. The disadvantage of this approach is it increases the series inductance of the line between the input and output ports and consequently reduces the high frequency cutoff off the low-pass filter section formed by this line.
The desirability of reducing the line inductance in band-stop filters has been generally recognized; however, it has not been as well recognized for bandpass filter. Attempts to reduce the line inductance by again substituting a wide strap for the usual single wire loop has resulted in the increases spurious response described previously. As an alternative to the strap, a number of parallel, closely spaced or touching wires has also been used with similar unsatisfactory results.
Practical prior art ferromagnetic resonator filters, which have been, for the most part, band-pass filters, usually make use of the same components described in connection with the band-stop filter of FIG. 4. That is, they make use of a coupling wire for coupling to the ferromagnetic material, which is typically in the shape of a sphere. The coupling wire is often formed into a loop around the sphere for increased coupling to the sphere. The coupling loops and spheres are housed in a structure which provides RF shielding between stages, but which allows the coupling wires to pass from stage to stage through the shield. A stage, as used herein is a basic filter section comprising for example one YIG sphere and its associated circuitry which is typically one or more coupling loops positioned about the sphere. In FIG. 2, loop 206 positioned about YIG sphere 201 comprise a first stage, loop 207 and sphere 202 comprise a second, and loop 208 and sphere 203 comprise a third.
Interstage coupling is accomplished with a wire with two loops formed along it, both ends of this wire being electrically connected to the RF housing or ground. Input and output coupling is accomplished with a wire loop connected from the input or output transmission line to the RF housing or ground. The coupling loops in each stage are positioned orthogonally with respect to one another to minimize coupling from one loop to another.
It is generally known that a band-pass filter can be designed by using input and output stage external Q and interstage coupling coefficients because the internal Q is relatively high in comparison to the Q of the external circuitry. The coupling values required are available from published tables and are related to the basic filter network. Thus, it is possible to attain the band-pass filter response desired from a ferromagnetic filter by adjusting the coupling loops close to the sphere, or by using a sphere which is large compared to the diameter of the coupling loop. Unfortunately, the spurious problem encountered with band-stop filters is present in band-pass filters as well. Both of these approaches to wide bandwidths lead to increased coupling to spurious modes. The spurious modes are undesirable in a band-pass filter because the tracking spurious modes produce additional passbands which degrades the filter out-of-band rejection and the crossing spurious modes produce additional passband insertion loss. Coupling to these spurious modes can be reduced by decreasing the unloaded Q of the ferromagnetic sphere, but this also causes increased passband insertion loss and a reduction in filter bandwidth. Thus, increased bandwidth in prior art devices results in degraded filter performance.